Just now connected a 680Ω resistor, CdS photo-cell [ #G15177 ] ,bright white LED [#G16402 ], & 16V power supply in a loop. In dark LED was out, full light LED was bright. Both LED & photo cell are verry non linear so they compliment ea. other verry well. First detectable light from LED was with one μA, and limited to 18 mA at full illumination by cell & 680Ω. Cell's range about 1 meg to 50Ω. Part nos from The Electronic Goldmine.